This application claims the benefit of Japanese Patent Application Number 2007-45948 filed on Feb. 26, 2007, the entirety of which is incorporated by reference.
1. Field of the Invention
The present invention relates to a servo motor controller (it will be called just as a controller in below) to control an operation of a servo motor provided on a machine tool or the like.
2. Description of the Background Art
In a general machine tool, a servo motor is rotated and driven so as to be able to slide a table or the like connected with a feeding shaft such as a ball screw or the like. In such a general machine tool, as positioning is made while the servo motor is reversibly rotating, an elastic deformation error, which is called as a lost motion, occurs at an elastic deformation unit of the feeding shaft or the like. Therefore, a controller in a machine tool has a correcting unit to correct the error. Here, a conventional controller in a machine tool (for example, as shown in the patent document 1) will be described referring to FIGS. 6 and 7. FIG. 6 is an explanation view illustrating a machine tool, and FIG. 7 is a block configuration diagram of a conventional controller 61.
As shown in FIG. 6, the machine tool includes a pair of brackets 72 and 72 fixed on a bed 71, and a ball screw 73 installed between the brackets 72 and 72. A nut 74 engaged with the ball screw 73 is attached with a table 75, and the table 75 is slid in the right and left directions by the ball screw 73 which is rotated by a servo motor 76. In addition, the ball screw 73 is held by the bracket 72 through a bearing which is not illustrated.
On the other hand, the controller 61 controls rotating and driving of the servo motor 76. The controller 61 assembles a position loop at which the deviation between an inputted NC position command and a position feedback signal from a position detector 62 provided at the servo motor 76 is made to be 0, and a speed loop using a speed feedback signal obtained by differentiating the position feedback signal. The controller 61 controls a torque command value of the servo motor 76 based on these two loops. Further, the controller 61 includes a forward-reverse detecting unit 63 to detect the rotating direction of the servo motor 76, and also includes a torque correcting unit 64 to correct a torque command value when the servo motor 76 is found to be reversed by the forward-reverse detecting unit 63.
In the above-described controller 61, a test run is preliminarily made and a value obtained by dividing the difference between a maximum value (TjA in FIG. 8(a)) and a minimum value (TjB in FIG. 8(b)) of a torque command value at the time of reversing by 2 is stored as a correction reference torque in a table corresponding to a feeding speed or the like. In an actual operation, when the forward-reverse detecting unit 63 detects reverse of the servo motor 76, the controller 61 reads a correction reference torque corresponding to a feeding speed at the time of reverse and corrects a torque command value. FIG. 8A illustrates changes of a torque command signal in the test run.
[Patent Document 1] Japanese Unexamined Patent Publication No. 1998-63325
The controller carries out correction based on a finally converging torque command value after the servo motor is reversed as illustrated in FIG. 8A. The correction is carried out with estimation of an elastic deformation error regarding that a torque is varied in a staircase pattern according to the reverse of the motor as illustrated in FIG. 8B. However, a torque does not actually reverse in a staircase pattern after the servo motor is reversed but gradually increases in the reversing direction so as to converge to a final torque value. Therefore, in the conventional controller, an estimation error occurs during the period from reversing of a servo motor to converging of a torque value to be a predetermined value, and thus an accuracy of positioning may decrease.
Further, in a case where a servo motor is reversed, there is a torque not working on an elastic deformation part such as a ball screw or the like, for example, rotation resistance in a bearing of a ball screw. Therefore, it is desired to estimate an elastic deformation error considering the torque not working on the elastic deformation unit or the like.